Add 3.10 support

Author: alippai

.github/workflows/tests.yml

@@ -34,7 +34,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        python: ['3.6', '3.7', '3.8', '3.9']
+        python: ['3.6', '3.7', '3.8', '3.9', '3.10']
     steps:
       - uses: actions/checkout@v1
       - name: Setup Build Tools

setup.cfg

@@ -13,6 +13,7 @@ classifiers =
     Programming Language :: Python :: 3.7
     Programming Language :: Python :: 3.8
     Programming Language :: Python :: 3.9
+    Programming Language :: Python :: 3.10
     Topic :: Software Development :: Libraries :: Python Modules
 keywords =
     httpx